DIY amp DI question

[GainFreak1984]

Hi all;

I'm looking to build a little direct box for recording and live sound purposes that I can connect directly to my speaker out. Here's the schematic of what I'd like to be working off of.

http://www.jensen-transformers.com/as/as066.pdf

First off, from everything I've researched, it seems that most boards want to see 600 ohms so I think I'm going to need to change over to a different transformer, and obviously make a few adjustments to the resistor values. Which brings me to the first question...

While I know Jensen makes some premium transformers, are there other acceptable readily available transformers that could be used with good results? Jensen also makes this one...
http://www.jensen-transformers.com/datashts/11flcf.pdf
which is noticeably less expensive but and I dont know if I really need that level of performance for a guitar amp.

My next questions I think are a little more critical...

The schematic shows C1 and C2 as a bipolar and a polypropylene in parallel on the input. Are both of these caps needed for some reason or can I just go with a single polypropylene cap? Also, what would be the prefered AC and DC ratings on these if I were to use it with a 100w amp running pretty close to flat out? The same rating question would hold for C3.

Lastly, it calls for a ceramic disc cap in the C4 position. There's a LOT of different types of ceramic caps out there and I'm completely confused as to which type I need to use here.

I'm pretty sure I'll have a few more questions as this little project progresses so thanks in advance for any help you guys can offer.

[dai h.]

if you want a line out tapped off the spk. out, then you want ones that look like this in that Jensen site's "Musical Instrument Application Schematics" :

http://www.jensen-transformers.com/as/as045.pdf

you could also forgo the transformer and just use some resistors to steal a bit of the spk. signal as in some amps.

as066 is not meant to be connected to the spk. out.

[GainFreak1984]

Some of the reasons why I picked the one i did was that the variable output level is very appealing along with the high cut function and ground lift. Also in the notes at the bottom of the schematic say that it can be used with high output signals such as guitar amplifiers as long as you engage the pad switch.

As far as using a couple resistors goes, that's pretty much what 90% of the amps on the market use for a line out and they almost universally sound like a sack O' s**t.

[dai h.]

okay, yeah I see that.

As far as using a couple resistors goes, that's pretty much what 90% of the amps on the market use for a line out and they almost universally sound like a sack O' s**t.

you mean sounding like s**t as in "doesn't sound like a spk. out"? That's because the signal needs to be EQ'ed to sound similar to a speaker. (That should be way more important than whether a transformer is used.) FWIW I have a Marshall SE100 and Palmer PDI-03 and they have stuff in there that filters the spk. signal (after it's dropped to line level). To my ear, they can sound okay but not really as satisfying as mic'ing a cab.

[Enzo]

In other words it sounds bad because it lacks speaker emulation, not because it lacks a transformer.

[GainFreak1984]

I'm fully aware that it sounds bad because of the lack of speaker emulation and THAT aspect has nothing to do with a transformer. The signal is going to get eq'd on the board for the house mix anyway. The reason I want to go with a transformer out is because I'm looking to get a true balanced output. I'm sure you could use an opamp in ummm ('differential' mode?) to get the same result but that would be a much bigger project than I'm up for. A tranny = comparatively simple, and not too many hassles involved.

[Enzo]

I am sorry, your somment about the resistor method always sounding bad led me to assume you were concerned over that and were using a tranny instead.

The op amp circuit is really quite simple, but it would require power supplies for the chip and stuff, so I wouldn't be adding one to a tube amp right away either.

For future reference regarding op amps, the easy way is to feed the signal to an op amp, and the output goes to one side of the balanced line. That output is also sampled back into the input of a second op amp, whose output is the complement. Inverting inputs of course.

[Steve Conner]

Did you try the red Behringer DI box with the built-in speaker simulator? I know Behringer are cheesy, but it's so cheap that it might be worth a gamble.

[Enzo]

Peavey makes one too, though I don't know what color it might be.

[dai h.]

the Behringer one (which I gather is the "Ultra-G GI100") looks pretty inexpensive--around 35 bucks(!). The single Jensen transformer probably costs about that much, lol...

[GainFreak1984]

I know there are several companies that make DI boxes and Lord knows they're all cheap enough. The whole point tho is to build it myself and maybe learn a little something in the process. Oh, BTW, Rolls makes one too that looks pretty close to identical to the Jensen circuit I posted.

Having said that, I'm back to my original questions regarding cap ratings and the ceramic disc cap composition. There's only about 5 million different ceramic caps listed in the Mouser catalog, and since this is clearly for RF and grounding protection I think I would want to use the absolute correct parts here.

[Steve Conner]

OK, well if you are determined to do it the hard way... :P

You cannot leave out C1, the bipolar electrolytic. The large capacitance is needed at this point in the circuit. However, you could leave out C2 across it. The practice of bypassing electrolytics with other kinds of capacitor is audiophoolery. If you want rid of the electrolytic, you would need to substitute a 22uF plastic film capacitor, which will be physically big and expensive.

The capacitors called out as "high quality polypropylene film" can probably be any plastic film you fancy.

C4 can probably be any ceramic or plastic film capacitor you have lying around.

About voltage ratings: the voltage rating of C1 doesn't matter because it's a coupling capacitor, so no signal voltage should appear across it. The lowest voltage of bipolar electro you can get will do.

As regards the other capacitors, the same argument would hold for C2, except we left it out. So that leaves us with C3 and C4. An amp cranking 100W into 16 ohms puts out about 60V peak. The resistors divide that by a factor of about 3/11, so a 50V capacitor will do fine for C3.

For C4, it all depends how much RF and line frequency hum you have around ;-) If it ever sees anything like 100V, something is wrong somewhere. I'd use any ceramic disc or plastic film cap that was handy. It doesn't actually have any safety-related function: that's what the green wires and ground pins on your amps and mixing boards are for.

PS: This is JM2C. I'm an electronic engineer in my day job, and I don't have time for all this stuff about different brands of capacitor having different tones. You asked for advice, but if you don't like mine, feel free to ignore it.

[GainFreak1984]

Cool, thanks for the explanation of where you got the voltage ratings and why. It helped a lot and made complete sense! And yeah, I'm determined to do it the hard way. Where's the fun in going out and buying something already built?!

As for line noise and hum, I don't know what kind of situations you see in the UK but out here when you play in a bar all bets are off as to what kind of power you're gonna be plugging into. At times it can be a real horror!

Aside from the flourescent beer signs, video games, ATM machines all on the same circuit that can make your equipment hum like mad, you can get ungrouned receptacles, reversed polarity, etc. Play out for a few years and you will have seen it ALL.

So, long story short, I've got a metric shit load of Mallory 150's over here that should work fine I guess. Use a decent bipolar and an NPO ceramic for ground lift and I should be good to go?

As for "tone caps" I think it's a total crock too. Cryogenically treated, aged, etc. I take that stuff as being in the same league as the 500 dollar speaker cables the audiophiles get all bent over. A fool and his money.....blah blah blah!